Conventional fuel systems for vehicles with internal combustion engines can include a canister that accumulates fuel vapor from a headspace of a fuel tank. If there is a leak in the fuel tank, the canister, or any other component of the fuel system, fuel vapor could escape through the leak and be released into the atmosphere instead of being accumulated in the canister. Various government regulatory agencies, e.g., the U.S. Environmental Protection Agency and the Air Resources Board of the California Environmental Protection Agency, have promulgated standards related to limiting fuel vapor releases into the atmosphere. Thus, it is believed that there is a need to avoid releasing fuel vapors into the atmosphere, and to provide an apparatus and a method for performing a leak diagnostic, so as to comply with those standards.
One technique in use for detecting fuel system leaks is known as “Natural Vacuum Leak Detection” (NVLD). In that method, the fuel system, including the fuel tank and canister, are sealed from the atmosphere immediately after an engine shut-down. Over time, vacuum develops in a fuel tank due to gas law effects, especially due to cooling of the tank. A vacuum switch changes state at a certain vacuum level, and that change in state is detected by a processor. If a sufficient vacuum (a sufficiently low pressure) is reached in the system to trip or maintain the switch in the vacuum state, then the system is deemed pass the leak test.
In the present specification, unless otherwise indicated, the term “pressure” means absolute pressure, and a pressure is said to “decrease” down to absolute zero pressure, or a “perfect vacuum.” A pressure is said to be “below” a threshold pressure if the pressure, in absolute terms, has a value less than the threshold pressure. That is true whether the pressures are above or below atmospheric pressure. In contrast, as used herein, the term “vacuum” denotes a pressure below atmospheric pressure; a vacuum is said to “increase” as it approaches absolute zero pressure, and a vacuum is said to “decrease” as it approaches atmospheric pressure.
A disadvantage of a conventional natural or passive vacuum evaporative leak detection system is that the testing pass/fail threshold is too low. That is to say, the leakage required to fail an evaporative leak detection test is relatively small. It is desirable for a test to yield a fail status when leakage is just below the required limit set by the various government regulatory agencies. That would maximize the opportunity to locate, and then repair, a system leak. This is particularly difficult in compact and sub-compact automobiles, which typically have small fuel tanks and tightly packaged underbody components.
The fuel tank leakage detection capability for many evaporative leak monitors is 0.5 mm (0.020″) as designated by the Air Resources Board of California. Some evaporative leak monitor applications that utilize the NVLD product have unnecessarily low pass/fail thresholds. For example, a system leak of only 0.25 mm (0.010″) is often large enough to trigger a malfunction indicator light (M.I.L.) using standard natural vacuum methods. That test outcome is considered to be type “alpha” error. An alpha error is an error caused by a “good” system failing the test. A measurement of alpha error for a fuel system leak detection (often expressed as a percentage) is:
      Alpha    ⁢                  ⁢    Error    =                              Number          ⁢                                                            ⁢                                                          ⁢          of          ⁢                                          ⁢          Leaks          ⁢                                          ⁢          detected          ⁢                                          ⁢          when          ⁢                                          ⁢          Leak                <                  0.5          ⁢                                          ⁢          mm                    ⁢                                  Number      ⁢                          ⁢      of      ⁢                          ⁢      Tests      ⁢                          ⁢      with      ⁢                          ⁢      8      ⁢      °      ⁢                          ⁢                        C          .                /        2            ⁢                                        ⁢                                      ⁢      hr      ⁢                          ⁢      Condition      
There is therefore presently a need to provide a method and system for decreasing the occurrence of alpha error by providing additional opportunities for the diagnostic to PASS even when medium system leakage exists. To the inventor's knowledge, no such technique is currently available.